Jonathan Borden, national sales manager at Brush Research Manufacturing (BRM)
If one reads the online forums, it is clear that identifying the optimum technique for removing the inevitable burrs created during machining of threaded parts is the subject of much debate. Internal threads, be they cut, rolled or cold-formed, can have burrs at hole entrances and exits, on thread crests, and on most slot edges. External threads on bolts, screws and spindles can also have burrs, particularly at the start of the thread.
For larger threaded parts, burrs can be removed by retracing the cutting path, but this increases the cycle time for each part. Secondary operations, such as heavy nylon deburring tools or butterfly brushes, can also be used. However, the challenges increase substantially if the diameter of the threaded part or tapped holes measures less than 0.125 in. This is because the microburrs created are sufficiently small that their removal requires polishing rather than aggressive deburring.
At this point, the choice of deburring solutions narrows considerably. Mass finishing techniques can be used, but these require the parts to be sent out, resulting in additional cost and loss of time. Many machine shops therefore prefer to keep secondary operations such as deburring in house by using hand drills, CNC machines or even manual techniques.
There are miniature brushes that, despite their tiny stems, filaments and overall dimensions, can be rotated using hand drills as well as adaptors on CNC equipment. These brushes are available in a range of sizes, tip styles and filament materials, including brass, carbon steel, nylon, abrasive-filled nylon, stainless steel and diamond abrasive.
Given the potential for burrs to affect the form, fit or function of a product, the stakes are high for products that have micro threads, for example, watches, eyeglasses, cell phones, digital cameras, printed circuit boards, precision medical devices and aerospace parts. Risks include misalignment of joined parts, difficulties in assembly, contamination of hygienic systems due to burrs becoming loose, and even fastener failure in the field.
Any displaced metal can cause critical and potentially hazardous situations in industries that rely on exceptional precision and cleanliness. Consequently, burr removal remains a critical finishing operation.
A BRM miniature brush for microburr removal
Mass finishing techniques for deburring
Mass finishing techniques, for example, tumbling, thermal deburring and electrochemical polishing, can be effective for removing some light burrs on small parts.
Tumbling can be used to remove some burrs, although it is generally not effective on the ends of threads. In addition, care must be taken to prevent the mashing of burrs into thread valleys, as this can interfere with assembly.
If the burrs are on internal threads, the mass finishing technique must be able to reach deep into internal structures. Thermal deburring utilises heat energy that approaches several thousand degrees Fahrenheit to attack burrs from all sides. The heat cannot transfer from the burr to the parent material, so the burr is only burned down as far as the parent material. As such, thermal deburring does not affect any dimensions, surface finish or material properties of the parent part.
Electrochemical polishing works my levelling out any burrs or micro-peaks. Although the technique is effective, there is still some concern it could affect the threads. Still, generally speaking, material removal conforms to the shape of the part.
Despite the potential issues, the low cost of mass finishing still makes it an appealing process for some machine shops. However, as already noted, machine shops prefer to keep secondary operations in house if possible.
A BRM miniature brush on a hand drill
Miniature deburring brushes
For threaded parts and machined holes less than 0.125 in, miniature brushes are an affordable tool to remove burrs and perform internal polishing. They are best suited to addressing tight tolerances, edge blending, deburring and other finishing requirements.
Machine shops often turn to using miniature brushes because they no longer want to outsource the work and are looking for a means to bring it in-house. Using miniature brushes means they no longer have to worry about the lead times and extra coordination involved to send parts out and bring them back.
Brush Research Manufacturing (BRM) offers miniature deburring brushes in a variety of sizes, tip styles and filament types. The company’s smallest diameter brush measures 0.014 in.
Miniature brushes can be used by hand. However, because the brush stem wires are very fine and may bend, use of a pin vice is recommended. BRM offers a double-end pin vice in kits of up to 12 brushes in both decimal (0.032–0.189 in) and metric (1–6.5 mm) hole sizes. The pin vice also allows the brushes to be rotated under power on handheld drills and CNC machines.
To prevent deflection of the twisted wire stem of the brush, CNC equipment can be programmed to apply the precise pressure and rotational speed. These types of deburring operations can be automated, even if one is using very small diameter miniature brushes. The tools on CNC machines can be used via either a pin vice or an adapter.
As mentioned previously, there are a number of miniature brushes available that vary not just in terms of size and tip-style but also filament type; brass, carbon steel, nylon, abrasive-filled nylon and stainless steel are commonly used.
Abrasive-filled nylon filaments can contain aluminum oxide, diamond abrasive or silicon carbide. The brushes are therefore particularly effective for removing burrs and polishing thread peaks and flank angles in tapped aluminum holes. If one cuts a single-point thread in aluminum or the part has been threaded using diamond tooling, there is a lot of fuzz and rough thread flank angles that need to be polished.
Stainless steel brushes are popular for more aggressive deburring of materials such as cast iron or steel in order to remove chips or clear breakthrough burrs. BRM’s smallest abrasive nylon brush is 0.032 in, but the company has been able to introduce three smaller stainless steel brush sizes, namely 0.014, 0.018 and 0.020 in, due to the nature of the material.
BRM also supplies diamond abrasive brushes for harder materials such as hardened steel, ceramic, glass and aerospace alloys.
The choice of filament depends on the surface finish specifications and if there is the need for more aggressive deburring. Other factors that apply to brushes used in automated applications include RPM of the machine tool, feed rates and optimum wear-life.
Although deburring of internal and external micro threads can be challenging, using the most suitable tools for a given application can simplify the task and assure all burrs are consistently removed on every part. In addition, by avoiding outsourcing of secondary deburring operations, machine shops can reduce turnaround time and price per part.
BRM
A BRM miniature brush kit